This invention relates to an optical display system and, more particularly, to an improved display of a B-scan ultrasonoscope.
The use of ultrasonic vibrational energy for sensing and displaying the shape and character of objects has long been known. More recently such ultrasonic detection and display systems have also been utilized for scanning of human tissue. Because the ultrasonic energy avoids damage to the issue, such scanning and display has been found especially useful in connection with sensitive portions of the body as, for example, the human eye.
While the transmission and reception of ultrasonic energy pulses is well known, when utilized in connection with human tissue, appropriate visual display becomes a major concern. In connection with human tissue scanning, not only are distance, shape and size of items of importance, but the density of the tissue is an important factor in the display.
In providing a suitable display, it has previously been known to present the reflected echoes on a picture tube either as the A-scan method or the B-scan method. In the A-scan technique, the transducer generates the pulse and is stationary with respect to the medium and the tissues contained therein. The echoes are then received and displayed as vertical deflections of the CRT tracing. The horizontal deflection is proportional to time and may be calibrated to indicate distance. The pulse stream displayed on the CRT screen will, therefore, represents an irregular "picket fence" with the height of the pickets indicating the strength of the echoes and the spacing between pickets showing depth information. In such display the brightness of the trace on the tube carries no information.
As a result, in order to achieve display of the geometrick arrangement of the tissue without losing reflectivity information, it is preferable to utilize the B-scan technique. In this method the transducer is moved across the tissue with the position of the vertical trace of the CRT being synchronized to follow the direction of the movement. The horizontal trace is again relative to time and, therefore, equivalent to distance. The brightness of the spots, however, indicates the echo strength and therefore the difference in acoustic impedance of the tissue is detected. In this technique a two dimensional or cross sectional picture of the anatomical structure being scanned is seen on the CRT screen.
Although the B-scan technique is preferable, visualization of the intensity or brightness is still limited because of the limitation of the CRT tube itself. The tissue echoes may cover a wide range of densities which may extend over 30 db or more. However, the brightness range of the CRT tube is limited to a narrower range, typically 18-20 db.
In order to solve this problem, it has heretofore been suggested to utilize a color display to achieve better recognition of the different densities of the tissue being scanned. One such prior art technique utilizing a color display has been described in U.S. Pat. No. 3,909,771 issued to the present inventors on Sept. 30, 1975, for "OPHTHALMIC B-SCAN APPARATUS". In that patent, there is described a technique for sending out ultrasonic pulses to the tissue and receiving the echo pulses which are reflected by appropriate structures in the tissue. These echo pulses are then converted to electrical signals which are then changed in digital values. The digital values represent incremental steps of a predetermined first continuous functional relationship with respect to the echo pulses received. The digital pulses are then converted into their binary coded decimal value and again converted into a second series of signals whose values represent incremental steps of another predetermined continuous functional relationship with the original echo pulses. These last series of signals are then respectively applied to color guns of a color display to appropriately display each signal in a preselected color.
Each of the predetermined continuous functional relationships can be selected in such a manner to accommodate the brightness range of the echo pulse intensity and also to accommodate the display such as the CRT tube.
While the aforementioned patent provides an improvement in the display by permitting visualization of tissue density, it has been found that the colors merge almost imperceptibly into each other and tissue interfaces are not easily seen.
Additionally, visual acuity is not very sharp and proper perception of the objects or bodies in the tissue causing the reflection is not clear. It is difficult to exactly determine the edges of the tissue and the visualization is not very sharp.